1. Field of the Invention
The present invention generally relates to electrical circuits and their fabrication. More particularly, this invention relates to a relatively uncomplicated process for forming thin-film metal resistors with improved dimensional tolerances.
2. Description of the Prior Art
Thin-film metal resistors have been employed in hybrid electronic circuits generally by vacuum deposition on small substrates. Nickel-based compositions such as nickel-chromium are often used, as well as other materials such as chromium silicide and tantalum nitride. Nickel-phosphorus thin film resistors have also been formed by lamination of large sheets of Ni-P-plated copper foil in printed circuit board constructions, with subsequent subtractive etching steps to define the resistors. These subtractive etching processes present the drawback of poor dimensional control due to the undercut effects of isotropic wet etching. To ensure a linear relationship between resistor aspect ratio (L/W) and resistance, a single resistor width is often used for all aspect ratios within a circuit, which imposes an undesirable restriction on the circuit layout or design. A second drawback of subtractive etching is that chemical etchants for the resistor metal are different from those typically employed in circuit board fabrication.
Accordingly, it would be desirable if a method were available that reduced the complexity of processing metal integral resistors while also promoting accurate control of resistance tolerances.